One of the most common machining requirements in metal working is that of making round holes. The most commonly accepted method for accomplishing this task is by use of a twist drill, a spade drill, an end mill or other tool that reduces the entire volume of the hole to chips. The rotating speed of such tools (RPM) is selected for the best cutting speed considering the material to be worked. The peripheral speed of the tool, which increases in direct proportion to the tool diameter, is the limiting factor for tool RPM. Hence, the tool rotation speed is selected to give the best metal cutting performance at the periphery of the bit, with the result that cutting efficiency necessarily declines toward the center of the hole. Since the cutting speed approaches zero at the hole center, the overall cutting efficiency is relatively low. The hole center is not cut by a drill, but is deformed outwardly to a point where it can be cut. The force required to do this must be provided by axial thrust on the drill. Thus, in drilling any given hole, production time and power consumption requirements are relatively high for the volume of chips generated and, a relatively high axial tool force is required.
The factors that affect the actual horsepower demand are the condition of the tool, the machinability of the material being cut, the efficiency of the machine tool and the rate of metal removal. Taking tool condition and the material being cut to be the same for any hole making process, the variable factors are machine tool efficiency and rate of metal removal. In most production scenarios, neither of these factors is particularly burdensome for conventionally drilled holes, but the larger hole diameters can cause problems, especially where multiple large holes are involved.
Trepanning, wherein a circular groove is cut to produce a round hole, is an alternative particularly suited to larger holes. In this operation, only the metal of the circular groove is cut and made into chips, so the power requirement is inherently much less than required for drilling. Trepanning can be preformed on a lathe, where the work piece rotates with respect to a stationary tool, or on a milling machine, with a rotating tool and a stationary work piece. Specialized cutting tools adapted to perform trepanning are commercially available for use with a drill press or vertical milling machine. These trepanning tools have a plurality of teeth around a hollow center and, like a drill bit, each tool makes only one hole size. A central slug of material remains uncut in the hollow center of the tool. Trepanning proportionately reduces the volume of chips to be cut and requires less power than conventional drilling. Not only is production cycle time reduced significantly, but also other advantages are present, such as the ability to make holes at an oblique angle to the surface of the material surface and an improved scrap value because of the solid central slug. The central slug must be contained within the length of the cutting tool and, when the hole is completed, the slug must be separated to allow its removal. This limits use of these tools to making through holes in plates no thicker than the tools axial length. In any case, the central slugs are useful as materials testing specimens.
Tool steel trepanning tools are commercially available but face difficulties in cutting harder alloy steels. Re-sharpening these tools is much more difficult than re-sharpening a conventional drill so that most shops send them back to the factory or to an authorized distributor to be reground. Trepanning tools with tungsten carbide cutting tips bonded on the cutting end are available but have also proven to have limited applications. The tungsten carbide cutting tips are necessarily thin and as a consequence are relatively fragile, with a tendency to shatter when used to cut hardened material. The broken toot is either sent back to the factory to be rebuilt or scrapped.
Therefore, the first object of the present invention is to provide an improved apparatus capable of machining holes and central slugs in hard materials. A second object is to provide such a tool in a form that is readily rebuilt by the user when necessary.